Abstract:In the present study, combined thermal desorption spectroscopy (TDS), microtomography and X-ray diffraction study has been carried out to identify the hydrogen trap sites in 7XXX aluminum alloys. Through constant heating rate TDS experiments, three distinct trap states have been identified. It is revealed that micropores are the predominant hydrogen trap site in alloys with medium hydrogen content, whereas grain boundaries is the major hydrogen trap site in alloys with low and high hydrogen content. We have cl… Show more
“…The dislocation densities were evaluated using the Williamson-Hall method 35 . The detailed procedure to estimate dislocation density was explained in our previous study 36 and the value was estimated to be 4.07 × 10 14 m −2 31 . The volume and surface area of all the pores present in the gauge region of the prepared alloys were analysed using the marching cubes algorithm 37 , where the three-dimensional morphology of pores can be measured by synchrotron X-ray tomography.…”
Aluminium alloys are re-evaluated as most feasible way to satisfy the industrial needs of light-weight structural materials. However, unlike conventional structural metals such as iron and titanium, aluminium does not have easily accessible secondary phases, which means that aluminium-based alloys cannot be strengthened by harnessing multiple phases. This leaves age hardening as the only feasible strengthening approach. Highly concentrated precipitates generated by age hardening generally play a dominant role in shaping the mechanical properties of aluminium alloys. In such precipitates, it is commonly believed that the coherent interface between the matrix and precipitate does not contribute to crack initiation and embrittlement. Here, we show that this is not the case. We report an unexpected spontaneous fracture process associated with hydrogen embrittlement. The origin of this quasi-cleavage fracture involves hydrogen partitioning, which we comprehensively investigate through experiment, theory and first-principles calculations. Despite completely coherent interface, we show that the aluminium-precipitate interface is a more preferable trap site than void, dislocation and grain boundary. The cohesivity of the interface deteriorates significantly with increasing occupancy, while hydrogen atoms are stably trapped up to an extremely high occupancy over the possible trap site. Our insights indicate that controlling the hydrogen distribution plays a key role to design further highstrength and high-toughness aluminium alloys.
“…The dislocation densities were evaluated using the Williamson-Hall method 35 . The detailed procedure to estimate dislocation density was explained in our previous study 36 and the value was estimated to be 4.07 × 10 14 m −2 31 . The volume and surface area of all the pores present in the gauge region of the prepared alloys were analysed using the marching cubes algorithm 37 , where the three-dimensional morphology of pores can be measured by synchrotron X-ray tomography.…”
Aluminium alloys are re-evaluated as most feasible way to satisfy the industrial needs of light-weight structural materials. However, unlike conventional structural metals such as iron and titanium, aluminium does not have easily accessible secondary phases, which means that aluminium-based alloys cannot be strengthened by harnessing multiple phases. This leaves age hardening as the only feasible strengthening approach. Highly concentrated precipitates generated by age hardening generally play a dominant role in shaping the mechanical properties of aluminium alloys. In such precipitates, it is commonly believed that the coherent interface between the matrix and precipitate does not contribute to crack initiation and embrittlement. Here, we show that this is not the case. We report an unexpected spontaneous fracture process associated with hydrogen embrittlement. The origin of this quasi-cleavage fracture involves hydrogen partitioning, which we comprehensively investigate through experiment, theory and first-principles calculations. Despite completely coherent interface, we show that the aluminium-precipitate interface is a more preferable trap site than void, dislocation and grain boundary. The cohesivity of the interface deteriorates significantly with increasing occupancy, while hydrogen atoms are stably trapped up to an extremely high occupancy over the possible trap site. Our insights indicate that controlling the hydrogen distribution plays a key role to design further highstrength and high-toughness aluminium alloys.
“…10) Reducing the hydrogen concentration in a material also leads to HE suppression. 11,12) However, to prepare a material with low hydrogen concentration, fabrication and thermal treatments must be performed under high vacuum conditions that are commercially not viable. 12) Recently, Tsuru et al revealed that the MgZn 2 precipitate interface spontaneously debonds as a result of hydrogen accumulation, and proposed this as a new mechanism for quasi-cleavage fracture in AlZnMg alloys.…”
Section: Introductionmentioning
confidence: 99%
“…11,12) However, to prepare a material with low hydrogen concentration, fabrication and thermal treatments must be performed under high vacuum conditions that are commercially not viable. 12) Recently, Tsuru et al revealed that the MgZn 2 precipitate interface spontaneously debonds as a result of hydrogen accumulation, and proposed this as a new mechanism for quasi-cleavage fracture in AlZnMg alloys. 13) It can be inferred that quasi-cleavage fracture is suppressed by reducing the hydrogen concentration at the MgZn 2 precipitate interface.…”
Recent studies have revealed that hydrogen embrittlement in AlZnMg alloys appears to be dominated by hydrogen partitioning to MgZn 2 precipitates. A method has recently been proposed for reducing the hydrogen concentration at MgZn 2 precipitates by adding specific intermetallic compound particles that have high hydrogen trap energy. In the present study, the effectiveness of Al 7 Cu 2 Fe particles on suppression of hydrogen embrittlement in AlZnMgCu alloys was evaluated using X-ray microtomography. Quasi-cleavage cracks were found to be initiated in regions where local volume fractions of the Al 7 Cu 2 Fe particles were relatively low. Hydrogen partitioning to the MgZn 2 precipitate interface was suppressed, even in high hydrogen concentration material, by adding Al 7 Cu 2 Fe particles. However, the fractional area of the quasi-cleavage fracture in the material with high hydrogen concentration was higher due to insufficient hydrogen diffusion inside the Al 7 Cu 2 Fe particles and at the interface between the aluminum matrix and the particles. It appears that finely distributed small Al 7 Cu 2 Fe particles might effectively suppress hydrogen embrittlement.
“…Al-Zn-Mg(-Cu) alloys are high-strength aluminum alloys that are widely used in aircraft and rolling stock, which require high strength-to-weight ratios. When the Zn or Mg content is increased to improve strength, susceptibility to stress corrosion cracking (SCC) increases, leading to delayed fracture [1][2][3] . Therefore, the SCC susceptibility of commercial alloys is suppressed both by adding trace elements such as Cr and Zr and by heat treatments such as overaging and retrogression-and-reaging treatments; the latter is a three-step heat treatment consisting of pre-aging, retrogression, and re-aging [4] .…”
Section: Introductionmentioning
confidence: 99%
“…According to Eqs. (1) and (2), the hydrogen partitioned to each trap site increases as the trap-site density and binding energy increase. Unlike the binding energy, the trap site density readily varies because of the thermal history and deformation [2,13,14,20] .…”
The local deformation and fracture behavior of high-Zn Al-Zn-Mg(-Cu) alloys under hydrogen influence were investigated by in situ tests through synchrotron X-ray tomography. Intergranular and quasi-cleavage fractures were induced by hydrogen, and strain localization by the presence of cracks was not observed by 3D strain mapping. This result suggests that the strain localization at the crack tip is smaller than the measurement limit of 3D strain mapping. The average crack-tip-opening displacements, which are one of the crack driving forces specified by fracture mechanics, directly measured from the tomographic slice were 0.14 and 0.23 m for intergranular cracks and quasi-cleavage cracks, respectively. The crack driving force of the intergranular and quasi-cleavage cracks was small. The local deformation behavior at the crack tips was analyzed based on fracture mechanics. The local deformation field of the crack tip, which was characterized using the Rice-Drugan-Sham (RDS) solution rather than the Hutchinson-Rice-Rosengren (HRR) solution, was located within 20 m of the crack tip, and its size was limited. The results of this work clarify that the intergranular and quasi-cleavage crack growth are caused by small driving forces; however, this behavior is not perfectly brittle, accompanying local deformation at the crack tip.
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